Introduced to Britain in the 1980s through the aquatic trade Hydrocotyle ranunculoides, commonly known as floating pennywort, is rapidly spreading through Europe and particularly in the UK, Belgium, Germany, Italy, France and the Netherlands. Originating in Central and South America, this stoloniferous perennial plant is forming dense, impenetrable mats which rapidly dominate water bodies, outcompeting and displacing native species and compromising flood defences, navigation and leisure activities. Despite its relatively recent introduction, establishment and spread have been exponential thanks largely to its extremely fast growth rate (up to 20cm per day) and its ability to re-generate from small fragments. In 2010, floating pennywort was added to section 14, schedule 9 of the UK’s Wildlife and Countryside Act 1981. A recent report estimates its cost to Great Britain’s economy as £25.5 million each year through management, disposal, flooding and indirect costs to boating and angling. News that from 2014 the sale of this plant will be banned is significant and welcomed.

As part of the EU Water Framework Directive, which requires all water bodies achieve good ecological status by 2015, CABI is investigating the potential for classical biological control of this problematic aquatic weed. Surveys in the plant’s native range (Argentina and Brazil) have revealed a particularly promising weevil, Listronotus elongatus, capable of overwintering in temperate climates. A shipment of weevils arrived in the UK in April this year and a resident population has been established in CABI’s quarantine facility.

Adult weevils feed mainly on the upper surface of the leaves producing characteristic feeding damage which can often lead to secondary pathogenic infection. The female lays eggs in small perforations in the petiole which the larva subsequently mines, moving into the submerged stolon, where it pupates causing the plant eventually to wilt and rot.

Extensive host range testing is currently underway against a carefully selected test plant list devised using recognised international protocols and agreed with botanists from the Natural History Museum; known as the centrifugal phylogenetic method (Wapshere, 1974) the list focuses on the most closely related species to the target weed in the area of introduction and gradually expands to include more distantly related plants so as to establish specificity. The genus Hydrocotyle belongs to the Araliaceae family and the test plant list comprises 79 species in total, representing members of the same family (including 2 UK natives) as well as the closely related family Apiaceae – the majority of these species are native to the UK and share similar aquatic habitats, whilst others are economically important crop and ornamental species. In addition, 30 less related but native species sharing a similar habitat are included.

Weevils have an excellent reputation for their specificity and have successfully tamed a number of aquatic plant menaces all over the world. Based on results to date, Listronotus elongatus continues to show promise as a closely co-evolved natural enemy of floating pennnywort and over the next year or so, we hope to be in a position to provide a comprehensive evaluation of its suitability for release into the environment in the UK, offering a safe, effective and sustainable long-term alternative to largely inadequate and expensive mechanical control methods.